What Is the Effect of Chlorine on pH in a Pool?

Evan Gillespie

Chlorine and pH levels are two of the most important pool water chemistry measurements. Although the two measurements are not directly linked, they can have an effect on one another in a variety of ways.

Ideal pH and Chlorine Levels

The pH level of pool water is measured on a scale of 0 to 14. A pH level below 7 indicates that the water is acidic, and a pH above 7 indicates that the water is basic. The ideal pH level for pool water is between 7.2 and 7.6; a pH level outside this range can cause swimmer discomfort, damage to the pool and ineffective disinfection.

The measurement of chlorine levels in pool water takes into account the amount of free chlorine, the type of chlorine that works as a disinfectant, and combined chlorine, chemical compounds that do not work as a disinfectant and are potentially harmful, in the water. Free chlorine levels should generally be between 1 and 2 parts per million, and combined chlorine levels should be less than 0.3 part per million.

Chlorine's Effect on pH

Different forms of chlorine pool disinfectants have different native pH levels, and when introduced into the pool in large doses, they may have a short-term effect on the water's pH level. Liquid chlorine and granular chlorine products have a high pH and may raise water pH, and some solid chlorine products have a low pH and may reduce water pH. The effect is temporary, however, and as the chlorine breaks down, the water will return to its initial pH level.

Chlorine and Water Testing

High levels of chlorine can interfere with the reagent used for pH testing in pool water test kits and give an inaccurate reading. When chlorine levels are above 10 parts per million, the reagent may turn purple rather than stay in the yellow to red range of an accurate test. If this happens, take a new water sample and add a drop of thiosulfate, which will neutralize the chlorine, before adding the reagent.

Chlorine Effectiveness and pH

More significant than the effect of chlorine on pH is the effect of pH on chlorine. Chlorine is only effective as a disinfectant when pH remains within the ideal range. When pool water pH is 7.2, chlorine is 63 percent effective, but when the pH rises to 7.8 percent, chlorine's effectiveness falls to 28 percent. When pH is 8.0, chlorine is only 3 percent effective.

At a very low pH level of 6.0, chlorine is 97 percent effective, but water with a pH that low is likely to cause damage to the pool and significant discomfort for swimmers.